T. Henningsen

Nonlinear optical characteristics of binary organic system

Single crystals of 3-nitroaniline-2-chloro-4-nitroaniline (m.NA-CNA) alloy were grown from the melt. The crystals do not show any damage when exposed to 1.06 μm radiation at 1 mJ per pulse (10 Hz repetition rate, 0.35 mm spot size, and 10 ns pulse) corresponding to a 100 MW/cm 2 power density. The second harmonic efficiency was measured by using a beam of 1.5 mm diameter spot 10 ns pulse and 1.06 μm wavelength. The input energy was 0.3 mJ, peak power density was 1.7 MW/cm 2 and the test crystal was 2 mm thick. The observed efficiency was 7.7% indicating that a crystal of 1 cm length will produce very high efficiency. This measured efficiency for the m.NA-CNA alloy is substantially higher than the reported value for the m.NA crystal

ZnGeP2 grown by the liquid encapsulated Czochralski method

The growth of ZnGeP2 by the liquid encapsulated Czochralski method is reported for the first time herein. Large boules of ZnGeP2, with diameters up to 40 mm and weights up to 400 gm were grown by Czochralski pulling from B2O3 encapsulated melts under high pressure (20 atm Ar) using axial gradients ≤120 °C/cm. Boules pulled at ≤4 mm/h exhibited large (50×20×15 mm3) monocrystalline grains of α‐phase ZnGeP2 with room temperature electrical properties of p‐type conduction, carrier concentrations ranging from 1012 to 1016 cm−3, and mobilities of 20 cm2/V s or less. Optical samples exhibited broad IR transmission (0.7 to 12.5 μm), second harmonic generation at 4.7 μm with 7.2% conversion efficiency, a broad subband gap photoluminescence signature, and near band‐edge absorption similar to that observed in Bridgman‐grown ZnGeP2.

Solution growth of vanillin single crystals

Single crystals of vanillin were grown by the solution growth method. Vanillin was observed to be very anisotropic, and crystal morphology was strongly dependent on solvent. Crystals grew in needle morphology in pure methanol and ethanol, and in plate morphology in pure chloroform. A mixture of methanol and chloroform was found to be a suitable solvent for crystal growth and crystals up to the size of 7 × 5 × 4 mm3 were grown for optical characterization. Optical quality of crystal was observed to be good. Preliminary studies on frequency conversion showed that vanillin has a higher figure of merit when compared to other crystals suitable for non-linear optical applications in the visible to near-infrared region.

Growth and characterization of gallium selenide crystals for far-infrared conversion applications

Abstract We have developed a method to synthesize large batch sizes of GaSe and have grown single crystals by the Bridgman method. Our results show a d value of 75 pV/m for GaSe. GaSe crystals were tested at a fluence of 1.6 J/cm 2 and 140 MW/cm 2 without damage. The 2 mm thick crystal was used in a high-power test where a crystal with an AR coating was able to operate at 30 kHz with an average power of 21 W into the crystal, equivalent to 32 kW/cm 2 CW in a 100 μm spot.

Growth and characterization of lead bromide crystals

Lead(II) bromide was purified by a combination of directional freezing and zone-refining methods. Differential thermal analysis of the lead bromide showed that a destructive phase transformation occurs below the melting temperature. This transformation causes extensive cracking, making it very difficult to grow a large single crystal. Energy of phase transformation for pure lead bromide was determined to be 24.67 cal/g. To circumvent this limitation, crystals were doped by silver bromide which decreased the energy of phase transformation. The addition of silver helped in achieving the size, but enhanced the inhomogeneity in the crystal. The acoustic attenuation constant was almost identical for the pure and doped (below 3000 ppm) crystals.

Growth and characterization of thallium arsenic selenide crystals for nonlinear optical applications

Abstract The synthesis, growth, and characterization of large single crystals of the efficient nonlinear optical sulfosalt material thallium arsenic selenide (TAS) are reviewed. Recent significant improvements in crystal growth and purification technology have enabled us to grow high optical quality crystals as measured by etchpit, x-ray topographic, and birefringent interferometric characterization methods. The excellent crystal quality is reflected in the high CO 2 frequency conversion efficiencies obtained with TAS. In addition to quality improvement in 2.5 cm diameter crystals, larger crystals, up to 5 cm in diameter, have now been grown by a modified Bridgman technique. Etching of crystals grown under a high axial thermal gradient indicated a higher defect density near the crystal surface than at the corresponding central region. In addition, high defect densities were found at the seed-crystal joint. This suggests that the etchpits are related to thermal stresses induced by the temperature distribution in a growing crystal. X-ray topographic analyses confirmed the etching studies, and also revealed a subgrain structure in some crystals which is related to the crystal stress distribution and to seeding defects. Birefringent interference patterns, a measure of optical uniformity, show a direct correlation between subgrain structure and optical homogeneity of the crystals. Improved growth control has eliminated these defects. The nonlinear optical susceptibilities of TAS are large, nearly three times those of proustite. Crystals grown in our laboratory have shown high values of second, third, and fourth harmonic conversion efficiency of the output from a CO 2 laser. Fifth harmonic generation has also been demonstrated. A 4.8 cm long crystal, 25 mm in diameter, was used to double 4.8 μm to 2.4 μm with an efficiency of 6.4% for the relatively low fluence of 8 mJ/cm 2 in an 80 nsec pulse.

Effect of growth conditions on the quality of lead bromide crystals

Single crystals of pure and doped lead bromide were grown by the Bridgman method in different convective conditions. The convection level was varied by changing the thermal and solutal Rayleigh number. The homogeneity in refractive index, and hence the optical quality, was estimated by examining the optical distortion, birefringence interferograms, and laser scattering through the crystal. The optical quality of the crystal varied significantly with the variation of convection level during the crystal growth. The critical concentration of the solute was estimated for several values of growth velocity by numerical analysis at the thermal gradient of 20 K/cm.

Effects of subgrain structures on the optical homogeneity of Tl3AsSe3 single crystals

Studies of subgrain structures in Tl3AsSe3 have been performed by x‐ray topography. Birefringent interference patterns show that a direct correlation exists between the subgrain structures and the optical homogeneity of the crystals. A quantitative measurement of lattice misorientation was performed from the interference patterns and compared with x‐ray rocking measurements. The origin of the subgrains is also discussed in detail.

Growth of subgrain-free Tl3AsSe3 crystals for efficient harmonic conversion to mid-infrared frequency

Abstract Minute crystal lattice misorientations, termed “subgrains”, degrade the optical homogeneity of Tl 3 AsSe 3 (TAS) crystals. Three origins of the subgrain formation were identified: variations in the seeding process, thermal stresses, and local adhesion of molten TAS to its growth ampoule. Systematic experiments and improved diagnostic techniques led to defect control and to the growth of TAS crystals free from subgrains along their entire crystal length. Key results were (1) the achievement of high optical quality, (2) demonstration of 57% efficient second harmonic generation from CO 2 laser radiation and (3) a significant reduction in optical absorption.

Thermosolutal convection during growth of organic nonlinear optical crystals

Single crystals of pure and binary alloy of m.dinitrobenzene and m.nitroaniline were grown by Bridgman method in a two zone transparent furnace. Effect of doping and growth velocity on the solid-liquid interface morphology and quality of crystal was determined by studying the optical transparency, birefringence and nonlinear optical characteristics.